It is conventional practice in the design of outdoor power tools to use a two-stroke internal combustion engine or an electric motor for powering an implement such as a line trimmer, a blower/vacuum or a chain saw. Two-stroke internal combustion engines are relatively light and may readily be carried by an operator during operation with various angular orientations. Two-stroke engines, however, have well-recognized exhaust emissions problems that often make them unfeasible for their use in areas that must comply with exhaust gas emissions regulations such as the California Air Resource Board (CARB) and Federal EPA regulations dealing with California air quality.
Limitations on the exhaust emission of carbon monoxide, hydrocarbons and oxides of nitrogen that will be required in the year 2000 cannot feasibly be met by outdoor power tools powered by using two-stroke internal combustion engines. Four-stroke internal combustion engines, on the other hand, provide a distinct advantage for outdoor power tool manufacturers in their attempt to meet the 2000 CARB emissions regulations. In addition, they operate quieter compared to two-stroke engines.
Unlike two-stroke engines which simultaneously admit a fresh charge of fuel and air mixed with lubrication oil while exhausting combustion products, including unburned fuel, a four-stroke internal combustion engine maintains the lubricating oil relatively isolated from the combustion chamber.
The intake and exhaust valve region and the camshaft and cam drive portions of a four-stroke internal combustion engine are lubricated with oil supplied from the crankcase. The combustion chamber remains relatively isolated from the crankcase. Lubricating oil is not introduced into the air/fuel mixture as in the case of a two-stroke engine. Lubrication of the cylinder wall occurs as a film of crankcase oil develops on the cylinder wall. The piston rings seal the variable volume combustion chamber and effectively prevent mixing of the lubrication oil with the combustion gases.
One disadvantage of using four-stroke internal combustion engines for outdoor power tools, aside from a weight disadvantage, traditionally has been the inability to operate the engine upside down or at the extreme tilt angles that would be required by the operator. Oil in the crankcase in those instances would tend to be drained through the engine block to the intake and exhaust valve region of the engine and would enter the air/fuel mixture intake flow path as well, thereby upsetting otherwise efficient fuel combustion during operation. Continuous operation of the engine, even for relatively short periods, may result in piston seizure because of an interruption of lubrication oil flow to the cylinder wall.
Many of the shortcomings associated with the use of four-stroke engines with outdoor power tools have been solved by the teachings of U.S. Pat. Nos. 5,241,932 and 5,421,292, which are incorporated in the present disclosure by reference. Those patents, which are assigned to the assignee of the present invention, disclose engines which avoid the weight disadvantage of using a four-stroke internal combustion engine in an outdoor power tool. Further, they make provision for efficient engine operation throughout a wide range of angular dispositions or orientations.
Copending U.S. patent application Ser. No. 08/614,835, filed Mar. 8, 1996, which also is assigned to the assignee of the present invention, describes a further improvement in the earlier prior art teachings. That improvement makes it possible to maintain distribution of lubrication oil mist throughout the engine while maintaining the liquid lubrication oil isolated from the valve chamber and the cam and cam drive system. The engine of the copending application, which is incorporated in the present disclosure by reference, includes a lubricating oil mist passage that is formed in the crankshaft and in the crankshaft counterweight. The crankshaft passage facilitates distribution of lubricating oil in the form of an oil mist through the region of the camshaft and the camshaft drive of the engine and through oil mist flow passages to an overhead valve chamber.
A lubricating oil mist generator driven by the crankshaft agitates liquid oil in the crankshaft and develops a lubricating oil mist which is transferred through the engine in a flow path defined in part by the crankshaft passage. Large oil droplets in the oil mist and liquid oil in the crankcase are prevented from entering the crankshaft passage by the effect of centrifugal force due to the rotation of the crankshaft and the crankshaft counterweight during engine operation. A fine oil mist, however, may pass through the crankshaft passage and through the engine lubrication system since the mist is relatively unaffected by the centrifugal forces created by the rotating crankshaft.